The plot begins with the decision of Alfred Ludens, a young history professor, to seek out his former mentor Vallar in the hope that he can restore his friend Patrick Fenman to health.

By the end of the novel Marcus Vallar has died, Ludens is no longer engaged to Irina, and Franca is still married to Jack, while Alison has left him.

The Message to the Planet, Iris Murdoch's twenty-fourth novel, continues the exploration of some of the themes familiar from her earlier works.

[5]: 207  Vallar, who is Jewish, becomes obsessed with the suffering of the Jews, reads books about the Holocaust, and visits Auschwitz concentration camp.

At the same time, several incidents and characteristics of Vallar recall the life of Jesus, including the apparently miraculous healing of Pat.

[5]: 209–210 The subplot concerning the relationship among Franca, Jack and Alison highlights another characteristic Murdoch subject, the egoism and selfishness inherent in erotic love.

[7] In his study of Iris Murdoch's fiction, Peter Conradi wrote that The Message to the Planet was a "lesser" novel, inferior to its predecessor The Philosopher's Pupil, but with some memorable scenes.